* [PATCH] accel/tcg: Add URL of clang bug to comment about our workaround
@ 2021-01-29 13:03 Peter Maydell
2021-01-29 15:19 ` Alex Bennée
0 siblings, 1 reply; 3+ messages in thread
From: Peter Maydell @ 2021-01-29 13:03 UTC (permalink / raw)
To: qemu-devel; +Cc: Paolo Bonzini, Richard Henderson
In cpu_exec() we have a longstanding workaround for compilers which
do not correctly implement the part of the sigsetjmp()/siglongjmp()
spec which requires that local variables which are not changed
between the setjmp and the longjmp retain their value.
I recently ran across the upstream clang bug report for this; add a
link to it to the comment describing the workaround, and generally
expand the comment, so that we have a reasonable chance in future of
understanding why it's there and determining when we can remove it,
assuming clang eventually fixes the bug.
Remove the /* buggy compiler */ comments on the #else and #endif:
they don't add anything to understanding and are somewhat misleading
since they're sandwiching the code path for *non*-buggy compilers.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
accel/tcg/cpu-exec.c | 25 +++++++++++++++++++------
1 file changed, 19 insertions(+), 6 deletions(-)
diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
index 6d017e46dd8..0abdae6f9f3 100644
--- a/accel/tcg/cpu-exec.c
+++ b/accel/tcg/cpu-exec.c
@@ -772,17 +772,30 @@ int cpu_exec(CPUState *cpu)
/* prepare setjmp context for exception handling */
if (sigsetjmp(cpu->jmp_env, 0) != 0) {
#if defined(__clang__)
- /* Some compilers wrongly smash all local variables after
- * siglongjmp. There were bug reports for gcc 4.5.0 and clang.
+ /*
+ * Some compilers wrongly smash all local variables after
+ * siglongjmp (the spec requires that only non-volatile locals
+ * which are changed between the sigsetjmp and siglongjmp are
+ * permitted to be trashed). There were bug reports for gcc
+ * 4.5.0 and clang. The bug is fixed in all versions of gcc
+ * that we support, but is still unfixed in clang:
+ * https://bugs.llvm.org/show_bug.cgi?id=21183
+ *
* Reload essential local variables here for those compilers.
- * Newer versions of gcc would complain about this code (-Wclobbered). */
+ * Newer versions of gcc would complain about this code (-Wclobbered),
+ * so we only perform the workaround for clang.
+ */
cpu = current_cpu;
cc = CPU_GET_CLASS(cpu);
-#else /* buggy compiler */
- /* Assert that the compiler does not smash local variables. */
+#else
+ /*
+ * Non-buggy compilers preserve these locals; assert that
+ * they have the correct value.
+ */
g_assert(cpu == current_cpu);
g_assert(cc == CPU_GET_CLASS(cpu));
-#endif /* buggy compiler */
+#endif
+
#ifndef CONFIG_SOFTMMU
tcg_debug_assert(!have_mmap_lock());
#endif
--
2.20.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] accel/tcg: Add URL of clang bug to comment about our workaround
2021-01-29 13:03 [PATCH] accel/tcg: Add URL of clang bug to comment about our workaround Peter Maydell
@ 2021-01-29 15:19 ` Alex Bennée
2021-02-09 10:20 ` Peter Maydell
0 siblings, 1 reply; 3+ messages in thread
From: Alex Bennée @ 2021-01-29 15:19 UTC (permalink / raw)
To: Peter Maydell; +Cc: Paolo Bonzini, Richard Henderson, qemu-devel
Peter Maydell <peter.maydell@linaro.org> writes:
> In cpu_exec() we have a longstanding workaround for compilers which
> do not correctly implement the part of the sigsetjmp()/siglongjmp()
> spec which requires that local variables which are not changed
> between the setjmp and the longjmp retain their value.
>
> I recently ran across the upstream clang bug report for this; add a
> link to it to the comment describing the workaround, and generally
> expand the comment, so that we have a reasonable chance in future of
> understanding why it's there and determining when we can remove it,
> assuming clang eventually fixes the bug.
>
> Remove the /* buggy compiler */ comments on the #else and #endif:
> they don't add anything to understanding and are somewhat misleading
> since they're sandwiching the code path for *non*-buggy compilers.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
--
Alex Bennée
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] accel/tcg: Add URL of clang bug to comment about our workaround
2021-01-29 15:19 ` Alex Bennée
@ 2021-02-09 10:20 ` Peter Maydell
0 siblings, 0 replies; 3+ messages in thread
From: Peter Maydell @ 2021-02-09 10:20 UTC (permalink / raw)
To: Alex Bennée; +Cc: Paolo Bonzini, Richard Henderson, QEMU Developers
On Fri, 29 Jan 2021 at 15:19, Alex Bennée <alex.bennee@linaro.org> wrote:
>
>
> Peter Maydell <peter.maydell@linaro.org> writes:
>
> > In cpu_exec() we have a longstanding workaround for compilers which
> > do not correctly implement the part of the sigsetjmp()/siglongjmp()
> > spec which requires that local variables which are not changed
> > between the setjmp and the longjmp retain their value.
> >
> > I recently ran across the upstream clang bug report for this; add a
> > link to it to the comment describing the workaround, and generally
> > expand the comment, so that we have a reasonable chance in future of
> > understanding why it's there and determining when we can remove it,
> > assuming clang eventually fixes the bug.
> >
> > Remove the /* buggy compiler */ comments on the #else and #endif:
> > they don't add anything to understanding and are somewhat misleading
> > since they're sandwiching the code path for *non*-buggy compilers.
> >
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Thanks. I'll put this in via target-arm.next unless somebody
has a preference for it going in via some other tree ?
-- PMM
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2021-02-09 10:22 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-01-29 13:03 [PATCH] accel/tcg: Add URL of clang bug to comment about our workaround Peter Maydell
2021-01-29 15:19 ` Alex Bennée
2021-02-09 10:20 ` Peter Maydell
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).